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Stop mounting ISO files in Linux with “-t iso9660”

Google “How do I mount an ISO image in Linux” and most of the links still say to use “-t iso9660”. For example:

mount -t iso9660 -o loop,ro diskimage.iso /mnt/iso

That worked fine 10 years ago, but these days not all ISOs use ISO9660 file systems. Many use the UDF (Universal Disk Format) file system, and if you specify ISO9660 when mounting a UDF ISO file, subtle problems can occur. For instance, file names that contain upper case letters on a UDF file system will appear in lower case when that ISO is mounted using ISO9660.

On any modern Linux distro mount is smart enough to figure out what type of file system to use when mounting an ISO file, so it’s perfectly fine to let mount infer the type, e.g.:

mount -o loop,ro diskimage.iso /mnt/iso

Here’s an example of what happens when you try to mount a type UDF ISO as type ISO9660. Note that the case of the file names changes to all lower case when mounting as iso9660, which in this case causes subtle errors to occur within the software.